The present invention relates to slag cement and more particularly to slag cement composed of ground granulated blast furnace slag.
Blast furnace slag is the non-metallic by-product of iron production, generally consisting of silicon, calcium, aluminum, magnesium and oxygen. When iron is manufactured using a blast furnace, two products collect in the hearth—molten iron and slag. The slag floats on top of the iron and is skimmed off to be fed to a granulator. In the granulator the molten slag is rapid quenched with water. The resulting granules are essentially glassy, non-metallic silicates and aluminosilicates of calcium. The glass content of the slag generally determines its cementitious character or suitability for use in a hydraulic cement—the higher the glass content the greater the cementitious properties.
Significant quantities of this blast furnace by-product are produced annually. Disposal of the blast furnace slag had been problematic until subsidiary uses for the slag were developed. For instance, granulated blast furnace slag (GBFS) may be added to cement clinker and calcium sulfate and inter-ground to create a modified portland cement. GBFS may also be added to the raw material feed stream for a cement kiln resulting in cement clinker. Where the GBFS is vitreous, additional yields of clinker have been obtained using the invention disclosed in U.S. Pat. No. 6,740,157, entitled “Method for cement clinker production using vitrified slag”, which issued on May 25, 2004, to the assignee of the present invention.
Ground granulated blast furnace slag (GGBFS) has been frequently used to replace a portion of portland cement in a concrete mixture. Blended cement (ASTM Type IS) is produced by inter-grinding and/or blending portland cement with the GGBFS. The GGBFS synergistically works with the portland cement to increase strength, reduce permeability, improve resistance to chemical attack and inhibit rebar corrosion.
Ground granulated blast furnace slag (GGBFS) has also been used as a separate cement component in its own right. Slag cement is a hydraulic binder, like portland cement, that reacts with water to form cementitious material (calcium-silicate hydrate). Grinding the GGBFS reduces the particle size to a cement fineness suitable for use as a hydraulic cement, which is typically less than 3500 cm2/g. In some cases, additives may be introduced to improve the physical properties of the slag cement, such as certain aluminosilicates, like fly ash, sulfate, and certain superliquifiers, such as naphthalene sulfonate.
Addition of GGBFS to portland cement in a blended cement and the replacement of portland cement with GGBFS take advantage of the energy invested at the foundry in the slag producing process, rather than at the cement mill. Grinding slag for cement replacement requires only about 25% of the energy needed to manufacture an equivalent amount of portland cement through the cement clinkering and milling process.
Blast furnace slag typically has a moisture content of 10-12% when it is received from a foundry. The slag is stockpiled, sometimes for several months, before being prepared for use as a clinker replacement or as slag cement. Often, the moisture content of this stockpiled slag reaches 20-22%. Thus, before the slag is used in a cement application it is first dried in a dryer operated at about 350° F. The energy required to dry the stockpiled slag results in an increase in the cost of the slag. The increased slag cost is still below the cost of producing a comparable amount of portland cement clinker. Although the slag cement is cheaper to produce than portland cement, customer acceptance of slag cement has stunted its usage, especially in the United States. From an economic standpoint, slag cement is not sufficiently less expensive than portland cement to drive the industry toward more slag cement usage, even though slag cement is acceptable for most cement applications.
Consequently, what is needed is a way to further reduce the cost of slag cement. This cost reduction should be accomplished without the need for significantly modifying existing slag cement processing equipment or necessitating the purchase of additional capital equipment. Thus, the cost reduction for the slag cement should be accomplished in a way that does not itself result in meaningful additional expense to the cement producer.